Problem: 10 rubber stamps cost $12.40. Which equation would help determine the cost of 3 rubber stamps?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. If we let $x$ represent the cost of 3 rubber stamps, we have the following proportion: $\dfrac{x}{3}$ We have to pay $12.40 for 10 rubber stamps, and that can be written as a proportion: $\dfrac{\$12.40}{10}$ Since the price per rubber stamp stays the same, these two proportions are equivalent. $\dfrac{x}{3} = \dfrac{\$12.40}{10}$